ticket 1600 - convert unittests to use DN objects
    
    We have a larger goal of replacing all DN creation via string
    formatting/concatenation with DN object operations because string
    operations are not a safe way to form a DN nor to compare a DN. This
    work needs to be broken into smaller chunks for easier review and
    testing.
    
    Addressing the unit tests first makes sense because we don't want to
    be modifying both the core code and the tests used to verify the core
    code simultaneously. If we modify the unittests first with existing
    core code and no regressions are found then we can move on to
    modifying parts of the core code with the belief the unittests can
    validate the changes in the core code. Also by doing the unittests
    first we also help to validate the DN objects are working correctly
    (although they do have an extensive unittest).
    
    The fundamental changes are:
    
    * replace string substitution & concatenation with DN object
      constructor
    
    * when comparing dn's the comparision is done after promotion
      to a DN object, then two DN objects are compared
    
    * when a list of string dn's are to be compared a new list is
      formed where each string dn is replaced by a DN object
    
    * because the unittest framework accepts a complex data structure of
      expected values where dn's are represeted as strings the unittest
      needs to express the expected value of a dn as a callable object
      (e.g. a lambda expression) which promotes the dn string to a DN
      object in order to do the comparision.